The invention relates to a heat exchanger, in particular an exhaust gas heat exchanger for motor vehicles.
DE-A 199 07 163 discloses a heat exchanger, in particular an exhaust gas heat exchanger for motor vehicles, which can be used in the exhaust gas recirculation (EGR) system as an exhaust gas cooler. The known exhaust gas heat exchanger is a welded stainless steel construction and has a housing jacket, a bundle of tubes with exhaust tubes and tube plates or header plates. The tubes are welded by their tube ends into punched-out openings in the header plates, and the header plates, for their part, are welded to the housing jacket. Hot exhaust gas flows through the tubes, and a liquid cooling medium, i.e., a coolant which is removed from the coolant circuit of the motor vehicle, flows around the tubes and within a jacket space inside of the housing jacket. The known exhaust gas cooler is intermittently subjected to hot exhaust gases, depending on whether an exhaust gas recirculation valve in an exhaust gas recirculation line is open or closed. The tubes assume the temperature of the hot exhaust gases while the housing jacket assumes the coolant temperature, which is substantially lower than the exhaust gas temperature. The above-mentioned welded connections between the tubes, header plates and housing jacket mean that the tubes are clamped on both sides fixedly in the housing jacket, i.e., the system is statically undetermined. The alternating action of the temperature on the exhaust gas tubes results in different expansions between the exhaust gas tubes and housing jacket, i.e., the tubes expand to a greater extent than the housing jacket and therefore cause thermal stresses, in particular in the region of the connections between the tubes and header plates. Added to this is the fact that the header plates bulge, i.e., are elastically deformed, because of the tube expansions, which means that the tubes are subject to a bending stress. Due to the manufacturing process, the header plates only have a maximum thickness of the order of magnitude of 1 to 2 mm, because the hole punching procedure employed to produce the openings causes problems in thicker header plates. The alternating bending stress on the tubes results in fatigue of the tube material in the region of the header plates and sometimes produces cracks in the tubes.